The right technology depends on what the part does, how many you need, what surface finish you need and how much you are willing to pay. We work only with SLS, so keep that in mind as you read. We have tried to describe the other technologies fairly, and if your part suits another process better, it is better to find out before you order.
How each technology works
FDM (filament extrusion)
A heated nozzle melts a plastic filament and lays it down in lines, layer by layer. Overhangs need printed supports, which are broken off afterwards. The layers bond to each other only through partial melting, so the bond between layers is the weak point of the part, and an FDM part breaks more easily along the Z axis than in the XY plane.
SLA (stereolithography)
A liquid resin cures layer by layer under a UV light source. The part is built on supports, then washed and post-cured. Of the 4 technologies, SLA gives the finest detail and the smoothest surface. The material, however, is a thermoset resin, unlike the thermoplastics used in FDM and SLS, and its mechanical behaviour depends heavily on the formulation you choose.
SLS (selective laser sintering)
A laser selectively melts a thin layer of polymer powder, usually polyamide (PA12, PA11), but also TPU or polypropylene. On Formlabs Fuse printers the layer is 110 µm thick. The unsintered powder stays around the part and supports it, so no support structures are needed. Parts can be stacked on top of each other throughout the build chamber.
MJF (Multi Jet Fusion, HP)
MJF also uses powder and also needs no supports. Instead of a laser, a print head deposits a black fusing agent, which absorbs infrared, on each layer, and a detailing agent along the contour. Infrared lamps then heat the whole layer at once, and the powder melts only where it received fusing agent. The layer is usually 80 µm thick. Because of the agent, parts come out dark grey and are usually dyed black.
Comparison by criterion
| Criterion | FDM | SLA | SLS | MJF |
|---|---|---|---|---|
| Supports | Yes | Yes | No | No |
| Material | Thermoplastic filaments (PLA, PETG, ABS, ASA, PC, nylon, composites) | Photopolymer resins | PA12, PA11, glass-filled or carbon-fibre-filled grades, TPU, PP | PA12, PA11, glass-filled PA12, TPU, PP |
| Surface | Visible layer lines | Smooth, almost glossy | Matte, slightly grainy | Matte, slightly grainy |
| Behaviour in Z | Noticeably weaker | Almost uniform | Somewhat weaker than in XY | Somewhat weaker than in XY |
| Colour | Any filament colour | Depends on the resin, clear is possible | Grey or white, can be dyed | Dark grey, usually dyed black |
| A simple one-off part | Cheapest | Medium | Medium to expensive | Medium to expensive |
| Runs of tens or hundreds | Slow, part by part | Limited by the build platform | Efficient, the chamber is filled in 3D | Efficient, the chamber is filled in 3D |
When FDM is the right choice
If you need a simple bracket, a box or a test part you will throw away after a day, FDM is usually the cheapest route. For large, solid parts, a large-format FDM printer can be cheaper than SLS. The range of filaments is very wide and includes materials we do not offer, such as ASA for outdoor use or polycarbonate. If you already have an FDM printer in the office and the part is not loaded along the Z axis, print it there.
FDM is a poor fit when the part has internal channels, clips that have to flex in every direction, thin walls or geometry that would need a lot of supports that are hard to remove. It is also a poor choice for 200 identical parts, because print time adds up part by part.
When SLA is the better choice
SLA gives better results on surface finish and fine detail. In a Formlabs study comparing surface finishes, the smoothest surface measured on an SLA part had an Ra of 0.5 µm, while bead-blasted PA12 SLS parts measured between about 8 and 16 µm, depending on the face. For appearance models, clear parts, moulds for casting, parts with small text or details under 0.5 mm, SLA is the better choice.
On the other hand, standard resins are more brittle than nylon, and the properties of some resins change over time with exposure to light and heat. Engineering resins exist (tough, heat-resistant, flexible), but each of them is a separate compromise. Supports also leave marks that have to be sanded by hand.
Similarities and differences between SLS and MJF
SLS and MJF are more alike than the marketing from their manufacturers suggests. Both use powder and need no supports. In both, parts can be stacked through the full height of the build chamber, and PA12 parts have comparable mechanical properties. In the same Formlabs study, the roughness of MJF PA12 parts ranged from 4.1 to 20.9 µm and that of SLS parts from 8.0 to 16.4 µm, so the ranges overlap.
The differences between them are mostly practical.
- Colour. The fusing agent is black, so standard MJF parts are dark grey and are dyed black. In SLS, PA12 comes out grey, while PA12 White comes out white and can be dyed in light colours.
- Production volume. Industrial MJF systems are built for high volumes. For thousands of identical parts a month, a large MJF service can offer a better price per part.
- Materials. The range differs from one manufacturer to another and from one service to another, so check that the material you need is available on the technology you choose. With us, for example, you can get PA11 CF, a polyamide filled with carbon fibre.
If you already have an MJF supplier you are happy with for PA12 parts, there is no significant technical reason to switch. In that case, the factors that decide are lead time, communication with the supplier and the distance to the workshop.
Questions to ask before you choose
- Does the part have to carry mechanical load, or only show what the product will look like? For appearance choose SLA, and for function choose SLS, MJF or a well-designed FDM part.
- What surface do you need? If the part has to feel smooth without post-processing, SLS is not the first choice.
- What temperature and load will it work under? PA12 has a heat deflection temperature of 87 °C at 1.8 MPa. For higher temperatures you need the reinforced grades.
- How many parts do you need now, and how many per year?
- Does it have internal geometry, channels or parts that have to move when printed fully assembled? For parts like these, powder-based technologies have a clear advantage.
When we do not recommend SLS
- Clear or glossy parts without post-processing.
- Details finer than 0.8 mm for pins or 1 mm for holes.
- Large, flat plates, which can curl during cooling, especially in PA11, TPU or PP.
- Parts that need a V-0 flammability rating, because our current materials are rated UL 94 HB.
- Simple one-off brackets that you can print yourself on an FDM printer.
If you are not sure which technology to choose, send us the part and tell us what it has to do. If SLS is not a good fit, we will tell you.
For SLS design limits see the SLS design guide, and for materials start with PA12. You can find example parts under functional prototypes. When your file is ready, you can send it for a quote.